Extracting carrier signals from modulated signals

ABSTRACT

Methods, systems, and apparatus for detecting a center frequency of an input signal, the input signal including a carrier signal modulated with a modulation signal. Detecting a frequency of a second signal. Determining a difference signal between the center frequency of the input signal and the frequency of the second signal. Modifying the frequency of the second signal based on the difference signal to provide the carrier signal. And, outputting the carrier signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 14/808,903, filed on Jul. 24, 2015, which is herebyincorporated by reference in its entirety.

BACKGROUND

Carrier modulation techniques are used to transmit information signalsfrom one location to another. Traditional signal modulation techniquesinclude, for example, amplitude modulation (AM), frequency modulation(FM), phase modulation (PM). In addition, complex modulation techniquesexist that incorporate aspects of AM, FM, and PM such as quadraturephase shift keying (QPSK), amplitude phase shift keying (APSK) andincluding quadrature amplitude modulation (QAM).

SUMMARY

This specification relates to methods and systems for transmitting andreceiving transpositional modulation (TM) signals. More specifically,the specification relates to methods and systems for extracting acarrier signal from an existing modulated signal, modulating the samecarrier signal with a TM signal, and combining the existing signal withthe TM signal. In addition, the specification relates to methods andsystems for receiving a combined traditional modulation and TM signal onthe same carrier signal and separating the TM signal from the combinedsignal. Although discussed in the context of TM, implementations of thepresent disclosure also may be applicable to transmitting and receivingsignals modulated using traditional types of modulation.

In a first general aspect, innovative aspects of the subject matterdescribed in this specification can be embodied in methods that includeactions of detecting a center frequency of an input signal, the inputsignal including a carrier signal modulated with a modulation signal.Detecting a frequency of a second signal. Determining a differencesignal between the center frequency of the input signal and thefrequency of the second signal. Modifying the frequency of the secondsignal based on the difference signal to provide the carrier signal.And, outputting the carrier signal. Other implementations of this aspectinclude corresponding systems, apparatus, and computer programs,configured to perform the actions of the methods, encoded on computerstorage devices.

This and other implementations can each optionally include one or moreof the following features. The carrier signal can be suppressed in theinput signal. The second signal can be generated by a signal generator.The signal generator can be a voltage controlled oscillator.

Modifying the frequency of the second signal can include using thedifference signal to control the signal generator. The method caninclude limiting an amplitude of the input signal. The method caninclude limiting an amplitude of the second signal. The method caninclude amplifying the difference signal.

Detecting the center frequency of the input signal can includemultiplying the input signal with a third signal to create a fourthsignal, and converting the fourth signal into a direct current (DC)voltage signal representing the center frequency of the modulated signalby integrating the fourth signal. Detecting the frequency of the secondsignal can include multiplying the second signal with a fifth signal tocreate a sixth signal, and converting the sixth signal into a directcurrent (DC) voltage signal representing the frequency of the secondsignal by integrating the sixth signal.

A second general aspect can be embodied in a device that includes asignal generator, a first frequency detector, a second frequencydetector electrically connected to the signal generator, the secondfrequency detector, and a difference circuit. The difference circuit iselectrically connected to the first frequency detector, to the secondfrequency detector, and to the signal generator. The first frequencydetector is configured to detect a center frequency of an input signal.The second frequency detector is configured to detect a frequency of anoutput signal of the signal generator. The difference circuit configuredto determine a difference signal from an output of the first frequencydetector and the second frequency detector, and to supply the differencesignal to the signal generator.

This and other implementations can each optionally include one or moreof the following features. The frequency of the output signal of thesignal generator can be controlled by the difference signal. The devicecan include a first amplitude normalizing circuit electrically connectedto the first frequency detector, and a second amplitude normalizingcircuit electrically connected between the signal generator and thesecond input of the difference circuit. The first amplitude normalizingcircuit can be configured to stabilize the amplitude of the inputsignal, and the second amplitude normalizing circuit can be to stabilizethe amplitude of the output signal of the signal generator. The firstamplitude normalizing circuit can be a first comparator and the secondamplitude normalizing circuit can be a second comparator.

The signal generator can be a voltage controlled oscillator. The firstfrequency detector can output a direct current (DC) signal representinga center frequency of a modulated input signal. The second frequencydetector can output a direct current (DC) signal representing afrequency of an output signal of the signal generator.

A third general aspect can be embodied in a device that includes aninput terminal for receiving a modulated signal; a signal generatorincluding a control input and an output, a first frequency detectorincluding an input and an output; a second frequency detector includingan input and an output; a difference circuit including a first input, asecond input, and an output, and an output terminal electricallyconnected to the output of the signal generator. The input of the firstfrequency detector is electrically connected to the input terminal. Theinput of the second frequency detector is electrically connected to theoutput of the signal generator. The first input of the differencecircuit is electrically connected to the output of the first frequencydetector, the second input of the difference circuit is electricallyconnected to the output of the second frequency detector, and the outputof the difference circuit is electrically connected to the control inputof the signal generator.

In a fourth general aspect, innovative aspects of the subject matterdescribed in this specification can be embodied in methods that includeactions of receiving an input signal, where the input signal includes acarrier signal modulated with a first modulation signal and a secondmodulation signal, and where the second modulation signal is a TMsignal. Demodulating the first modulation signal from the input signal.Modulating an un-modulated carrier signal with the first modulationsignal to generate a third signal, where the third signal includes thecarrier signal modulated by the first modulation signal. And, removingthe first modulation signal from the input signal by subtracting thethird signal from the input signal to extract the TM signal from theinput signal. Other implementations of this aspect include correspondingsystems, apparatus, and computer programs, configured to perform theactions of the methods, encoded on computer storage devices.

This and other implementations can each optionally include one or moreof the following features. The method can include providing the TMsignal to a TM demodulator. Demodulating the first modulation signal caninclude performing in-phase and quadrature phase demodulation.Modulating the carrier with the first modulation signal can includeperforming in-phase and quadrature phase modulation. The firstmodulation signal can be one of phase modulation, frequency modulation,binary phase shift keying, quadrature phase-shift keying, amplitude andphase-shift keying, or quadrature amplitude modulation.

The method can include filtering the first modulation signal. The methodcan include delaying the input signal. Demodulating the first modulationsignal from the input signal can include mixing the input signal withthe un-modulated carrier signal, and modulating the un-modulated carriersignal with the first modulation signal to generate a third signal caninclude delaying the un-modulated carrier, and modulating the delayedun-modulated carrier signal with the first modulation signal to generatethe third signal.

A fifth general aspect can be embodied in a device that includes ademodulator configured to demodulate a first modulation signal from theinput signal, where the input signal includes a carrier modulated withthe first modulation signal and a TM signal, a modulator electricallyconnected to the demodulator and configured to modulate the carrier withthe first modulation signal to generate a third signal that includes thecarrier modulated by the first modulation signal, and a differencecircuit electrically connected to the modulator and configured to removethe first modulation signal from the input signal by subtracting thethird signal from the input signal.

This and other implementations can each optionally include one or moreof the following features. An output of the difference circuit can beelectrically connected to a TM signal demodulator. The demodulator canbe an in-phase and quadrature phase demodulator. The modulator can be anin-phase and quadrature phase modulator. The first modulation signal canbe one of phase modulation, frequency modulation, binary phase shiftkeying, quadrature phase-shift keying, amplitude and phase-shift keying,or quadrature amplitude modulation.

The device can include a filter electrically connected to thedemodulator and the modulator. The device can include a delay circuitelectrically connected to the difference circuit and configured to delaythe input signal. The demodulator can receive an first un-modulatedcarrier signal as input, and the modulator can receive a secondun-modulated carrier signal as input. The second un-modulated carriersignal can be delayed from the first un-modulated carrier signal.

A sixth general aspect can be embodied in methods that include actionsof receiving an input signal, the input signal including a carriersignal modulated with a first modulation signal and a second modulationsignal where the second modulated signal is in quadrature with respectto the first modulation signal. Demodulating the first modulation signalfrom the input signal. Modulating an un-modulated carrier signal withthe first modulation signal to generate a third signal, where the thirdsignal includes the carrier signal modulated by the first modulationsignal. And, removing the first modulation signal from the input signalby subtracting the third signal from the input signal to extract thesecond modulation signal from the input signal. Other implementations ofthis aspect include corresponding systems, apparatus, and computerprograms, configured to perform the actions of the methods, encoded oncomputer storage devices.

Particular implementations of the subject matter described in thisspecification can be implemented so as to realize one or more of thefollowing advantages. Implementations may increase the bandwidth ofsignals transmitted using traditional modulation schemes.Implementations may permit the combination of two differently modulatedsignals on a single carrier frequency. Some implementations may permitextraction of carrier signals from a modulated signal with little or noa priori information about the modulated signal. Some implementationsmay be capable of extracting a carrier from a modulated signal withoutregard to the type of modulation used in the modulated signal. In otherwords, some implementations may able to extract carrier signals whilebeing agnostic to the type modulation of an input signal.

The details of one or more implementations of the subject matterdescribed in this specification are set forth in the accompanyingdrawings and the description below. Other features, aspects, andadvantages of the subject matter will become apparent from thedescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an example system in accordance with implementations ofthe present disclosure.

FIG. 2 depicts a block diagram of an example TM signal transmitter inaccordance with implementations of the present disclosure.

FIG. 3A depicts a block diagram of an example carrier extractor inaccordance with implementations of the present disclosure.

FIG. 3B depicts a block diagram of an example frequency detector inaccordance with implementations of the present disclosure.

FIGS. 4A and 4B depict example control signals generated by a carriersignal extraction device.

FIG. 5 depicts a block diagram of an example TM signal receiver inaccordance with implementations of the present disclosure.

FIG. 6A depicts a block diagram of an example TM signal separation andextraction device in accordance with implementations of the presentdisclosure.

FIG. 6B depicts frequency domain representations of signals at variousstages of the TM signal separation and extraction device shown in FIG.6A.

FIGS. 7 and 8 depict example processes that can be executed inaccordance with implementations of the present disclosure.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

Implementations of the present disclosure generally extract a carriersignal from an existing modulated signal, modulate the extracted carriersignal with a transposition modulation (TM) signal, and combine andretransmit the existing signal with the TM signal on the same carriersignal. Specifically, the implementations can extract a carrierfrequency from a modulated signal in which the carrier signal has beensuppressed (e.g., QPSK, QAM, APSK, BPSK). A CAREX (carrier extraction)circuit determines a frequency difference between the frequency of theCAREX output signal and a weighted average of the carrier frequency ofthe input signal. The calculated difference value is used tocontinuously tune a signal generator to maintain a minimal differencebetween the weighted average of the input carrier frequency and theCAREX output. The third harmonic of the extracted carrier is modulatedwith a data signal generating a TM modulated signal. The TM modulatedsignal is heterodyned back to the extracted carrier frequency andcombined with the existing modulated signal. The combined signal canthen be transmitted. Moreover, the TM modulated signal in the combinedsignal does not interfere with the existing signal because the TMmodulation is not recognized by demodulation systems used to demodulatetraditional modulation schemes. Instead, the TM signal appears as aslight increase in noise within the existing signal.

Other implementations of the present disclosure generally receive acombined traditional modulation and TM signal on the same carrier signalthen separate the TM signal from the combined signal. Specifically, theimplementations can separate the existing signal from a combined signalincluding a traditionally modulated signal (the existing signal) and aTM modulated signal. The existing signal can be demodulated from thecarrier signal. An extracted carrier signal can be re-modulated with thedemodulated existing signal to re-create the existing signal alone,absent the TM modulated signal. The re-modulated existing signal can beremoved from the combined signal leaving only the TM modulated signalwhich can be demodulated using TM demodulation techniques describedherein.

FIG. 1 depicts an example system 100 in accordance with implementationsof the present disclosure. The system 100 is a system of signaltransmitters and receivers 102. The system 100 may be a radio frequency(RF) communication system, a satellite communication system, a landlinecommunication system (e.g., a telephony or cable network), an opticalcommunication system, or any other system of signal transmitters andreceivers 102. The transmitters/receivers 102 include systems formodulating a carrier signal with an information signal using traditionalmodulation techniques and transmitting the modulated signal from onetransmitter/receiver to another. Traditional modulation techniquesinclude, for example, amplitude modulation (AM), frequency modulation(FM), and phase modulation (PM) in addition to complex modulationtechniques that incorporate aspects of AM, FM, and PM such as quadraturephase shift keying (QPSK), amplitude phase shift keying (APSK) andincluding quadrature amplitude modulation (QAM). In addition,transmitter/receiver B includes a TM transmitter 104 andtransmitter/receiver C includes a TM receiver 106.

The system 100 can receive a traditionally modulated signal 108 andcombine the traditionally modulated signal 108 with a TM modulatedsignal 110 on the same carrier using a TM transmitter 104, therebyincreasing the overall bandwidth of the combined signal 112. The TMmodulated signal 110 can be separated from the combined signal 112 anddemodulated by a TM receiver 106. Likewise, the traditionally modulatedsignal 108 can be separately demodulated with no interference caused bythe TM modulated signal 110. This is possible because TM modulatedsignals are undecipherable by non-TM receivers, instead appearing as aslight increase of noise in traditionally modulated signals.

For example, transmitter/receiver A may transmit a QAM signal 108 totransmitter/receiver B. The TM transmitter 104 at transmitter/receiver Bcan receive the QAM signal 108 and extract the carrier signal from theQAM signal 108. The TM transmitter 104 modulates the extracted carriersignal with a TM signal, combines the TM modulated signal 110 with theQAM signal 108, and retransmits the combined signal 112. In someexamples, as described below, the TM transmitter 104 can extract acarrier signal from a traditionally modulated signal 108 (e.g., the QAMsignal) in which the carrier is suppressed and while having little or noa priori information about the carrier signal (e.g., frequency or phaseinformation).

Transmitter/receiver C can then receive the combined signal 112. The TMreceiver 106 of transmitter/receiver C separates and extracts the TMmodulated signal 110 from the combined signal 112, and then demodulatesthe TM modulated signal 110 to obtain the TM modulated data signal. Insome examples, as described below, the TM receiver 106 separates the TMmodulated signal 110 from the combined signal 112 by demodulatingtraditionally modulated signal 108 (e.g., the QAM signal), re-modulatingthe carrier with only the traditionally modulated signal 108, andsubtracting the re-modulated carrier signal from the combined signal 112leaving only the TM modulated signal 110.

In some implementations, the carrier signal can be an intermediatefrequency (IF) carrier signal. That is, the carrier signal is notnecessarily at the same frequency of the carrier upon which the signalis ultimately be transmitted, but may be at an IF used internally withina system (e.g., a satellite communication system) as an intermediatestep in either signal transmission or reception. That is, in the case ofsignal transmission, a system may up-convert a combined signal 112 fromthe IF signal to a transmission carrier frequency prior to transmittingthe combined signal 112. Conversely, in the case of signal reception, asystem may down-convert a modulated signal from the transmission carrierfrequency to an IF frequency before separating the TM modulated signal110 from the combined signal 112. In other implementations, an IFcarrier signal may not be used, and the transmission carrier signal canbe modulated with both a traditionally modulated signal and a TMmodulated signal.

FIG. 2 depicts a block diagram of an example TM signal transmitter 104in accordance with implementations of the present disclosure. The TMtransmitter 104 includes a carrier extraction portion (CAREX 206), aharmonic generation portion 202, a TM modulating portion 204, and aheterodyning portion 205. The carrier extraction portion includes thecarrier extractor (CAREX) 206. The harmonic generation portion 202includes a second harmonic generator 208 and a third harmonic generator210. The TM modulating portion 204 includes a signal optimizer 212 and aTM modulator 214. And, the heterodyning portion 205 includes a signalmixer 216, a bandpass filter 218, and a power amplifier 220. Inaddition, the TM transmitter 104 includes a signal coupler 222 and asignal combiner 224.

In operation, the TM transmitter 104 receives an existing modulatedsignal (e.g., traditionally modulated signal 108 of FIG. 1). The signalcoupler 222 samples the existing modulated signal and passes the sampleof the existing modulated signal to the CAREX 206. The CAREX 206extracts a carrier signal (f_(c)) from the existing modulated signal.The CAREX 206 is described in more detail below in reference to FIGS.3A-4B. The output of the CAREX 206 is a pure sinusoidal signal at thefundamental frequency of the carrier from the existing modulated signal.In some examples, the CAREX 206 is agnostic to the type of modulationused in the existing modulated signal. That is, the CAREX 206 canextract the carrier signal from an existing modulated signal regardlessof the type of modulation used in the existing modulated signal. In someexamples, the CAREX 206 can extract carrier signals even when thecarrier is suppressed in the existing modulated signal, and can do sowith little or no a priori information about existing modulated signal'scarrier (e.g., frequency or phase modulation information).

The CAREX 206 passes the extracted carrier signal to a second harmonicsignal generator 208 and a third harmonic signal generator 210, whichgenerate signals at the second and third harmonic frequencies (2 f _(c)and 3 f _(c) respectively) of the fundamental carrier frequency (f_(c)).The second and third harmonic signals (2 f _(c), 3 f _(c)) are used bythe TM modulation portion 204 and the heterodyning portion 205 of the TMtransmitter 104 to generate a TM modulated signal and to heterodyne theTM modulated signal to the fundamental carrier frequency (f_(c)).

The TM modulation portion 204 of the TM transmitter 104 modulates thethird harmonic (3 f _(c)) of the carrier signal (f_(c)) with a datasignal to generate the TM modulated signal. The TM modulated signal isthen heterodyned to the frequency of the carrier signal (f_(c)),combined with the existing modulated signal, and outputted to an antennafor transmission.

In more detail, TM modulation portion 204 receives a data signal fortransmission (e.g., a baseband (BB) data signal). The BB data signal canbe, for example, an analog signal that may be quantized to conveydigital bits. The data signal is optionally processed for transmissionas a TM modulated signal by the signal optimizer 212. In some examples,the signal optimizer 212 produces an optional pattern of inversion andnon-inversion of the modulating signal, and filters the modulatingsignal to ensure that the total bandwidth of the data signal is withinthe channel bandwidth of the existing modulated signal. In someexamples, the signal optimizer 212 can include sample-and-hold circuitryand filters to prepare the modulating signal for transmission as a TMmodulated signal. In some examples, the signal optimizer 212 can bebypassed or turned off and on.

The TM modulator 214 modulates the third harmonic (3 f _(c)) of thecarrier signal (f_(c)) with a data signal to generate the TM modulatedsignal. For example, the TM modulator 214 modulates the third harmonic(3 f _(c)) by introducing a variable time delay based on the datasignal. In other words, the TM modulator 214 can use the data signal asa control signal for introducing an appropriate time delay to thirdharmonic (3 f _(c)). As such, an amount of time delay introduced intothe third harmonic (3 f _(c)) represents discrete bits or symbols of thedata signal. The described time delay modulation technique may beconsidered as time-shift modulation and is performed on the thirdharmonic (3 f _(c)) of the intended carrier frequency (3 f _(c)).

The time-shift modulation of the third harmonic (3 f _(c)) produces asingle set of upper and lower Bessel function sidebands. The inventorhas confirmed such results in laboratory simulations with anoscilloscope and spectrum analyzer. Moreover, the bandwidth of thesesidebands can be limited to the bandwidth of an intended communicationchannel by the optimizer 212 before TM modulation of the signal, asdescribed above.

In some examples, the time delay may be a phase shift. However, thetime-shift modulation described above is not equivalent phasemodulation. As noted above, the inventor has confirmed in laboratorytests that the time-shift modulation only produces a single pair ofupper and lower Bessel sidebands. Phase modulation, however, produces aseries upper and lower Bessel sidebands.

The heterodyning portion 205 prepares the TM modulation signal do becombined with the existing modulated signal and transmitted by thereceiver. The TM modulated signal is then heterodyned (e.g., frequencyshifted) by mixer 216 down to the fundamental frequency of the carriersignal (f_(c)). The mixer 216 multiplies the TM modulated signal withthe second harmonic of the carrier (2 f _(c)) which shifts the TMmodulated signal to both the fundamental carrier signal frequency(f_(c)) and the fifth harmonic frequency of the carrier. The bandpassfilter 218 removes signal at the fifth harmonic frequency as well as anyadditional signals or noise outside of the bandwidth of the TM modulatedsignal centered at the fundamental carrier signal frequency (f_(c)).

The TM modulated carrier signal is amplified by power amplifier 220 andcombined with the existing modulated signal by the signal combiner 224.It may be necessary, in some examples, to adjust the phase of the TMmodulated carrier signal to match the phase of the carrier in theexisting modulated signal before combining the two signals.

FIG. 3A depicts a block diagram of an example CAREX 206 in accordancewith implementations of the present disclosure. The CAREX 206 can beimplemented as a circuit in a device such as a TM transmitter or TMreceiver, for example. In some implementations, the CAREX 206 can beimplemented as a standalone device for installation into in a largersystem (e.g., an application specific integrated circuit (ASIC) or fieldprogrammable logic array (FPGA)). In some implementations, the CAREX 206can be implemented in software, for example, as a set of instructions ina computing device or a digital signal processor (DSP).

The CAREX 206 operates by determining a center frequency of an inputsignal (e.g., either modulated or unmodulated), comparing the centerfrequency to the frequency of a pure sinusoidal signal produced by asignal generator to create an error signal, and adjusting the frequencyof the signal generator output signal based on a control signalgenerated from the error signal until the error signal is minimized.Furthermore, the CAREX 206 does not require a priori information about acarrier signal to extract the carrier signal and can extract carriersignals when the carrier of the modulated signal is suppressed.

The CAREX 206 includes amplitude limiters 302 a, 302 b, filters 304 a,304 b, frequency detectors 306 a, 306 b, signal generator 308,difference circuit 310, and an amplifier 312. The amplitude limiter 302a and filter 304 a condition input signal before the input signal isanalyzed by the first frequency detector 306 a. The amplitude limiter302 a removes any variations in the amplitude of the input signal. Inother words, the amplitude limiter 302 a stabilizes the amplitude of theinput signal. In some examples, the amplitude limiters 302 a, 302 b canbe an analog comparator or an automatic gain control (AGC) circuit. Thefilters 304 a, 304 b are bandpass filters and removes extraneous signals(e.g., harmonics) and noise outside the channel bandwidth of the inputsignal.

The frequency detectors 306 a and 306 b can be frequency discriminatorsor quadrature detectors. The first frequency detector 306 a detects thecenter frequency of the input signal. As shown in the frequency domainplot 320, an input signal produced by traditional modulation techniquesgenerally has symmetric sidebands 322 located on either side of thecarrier frequency 324. The frequency detector 306 a can determine acenter frequency of an input signal based on, for example, thefrequencies of the outer edges of the sidebands 322. Furthermore, thefrequency detector 306 a can use the sidebands 322 of an input signal todetermine the center frequency even if the carrier signal 324 issuppressed, as illustrated by the dotted line.

The signal generator 308 generates a pure sinusoidal signal (e.g., asingle frequency signal) which is provided to a second frequencydetector 306 b. The signal generator 308 can be, for example, a voltagecontrolled oscillator (VCO) such as, but not limited to, a voltagecontrolled LC (inductor-capacitor) oscillator circuit, a voltagecontrolled crystal oscillator (VCXO), or a temperature-compensated VCXO.The second frequency detector 306 b detects the frequency of the outputsignal from the signal generator 308. In some examples, the outputsignal from the signal generator 308 is provided to an amplitude limiter302 b and filter 304 b before being transmitted to the second frequencydetector 306 b. The amplitude limiter 302 b and filter 304 b stabilizeand filter the amplitude of the signal generator output signal similarto amplitude limiter 302 a and filter 304 a.

The output from each of the first and second frequency detectors 306 a,306 b is provided as inputs to the differencing circuit 310. The outputof both the first and second frequency detectors 306 a, 306 b can be, insome examples, a direct current (DC) voltage signal representing thecenter frequency of the input signal and the frequency of the signalgenerator 308 output signal, respectively. The output of the differencecircuit 310 is an error signal representing the difference in frequencybetween the center frequency of the input signal in the signal generatoroutput signal. The error signal (e.g., a DC voltage) is amplified byamplifier 312 and provided as a control signal to the signal generator308. The amplifier 312 can be, for example, a high gain integratingcircuit that integrates the inputted error signal over time to producethe control signal.

The signal generator 308 adjusts the frequency of its output signalbased on the control signal until the frequency of the signal generator308 output is matched to the center frequency of the input signal. TheDC value of the control signal is used to control the frequency of thesignal generator output, as shown in FIG. 4B and described below. Thesignal generator output is provided as the output of the CAREX 206.Frequency domain plot 330 and time domain plot 334 represent an exampleCAREX 206 output signal. As shown, the output signal of the CAREX 206 isa pure sinusoidal signal having a frequency 332 equivalent to thefundamental carrier frequency of the input signal.

In some implementations, the frequency detectors 306 a and 306 b arematched. In some examples, the matched frequency detectors 306 a and 306b have similar frequency to DC output characteristics over changingmodulated input frequencies. In some examples, the matched frequencydetectors 306 a and 306 b have similar thermal and aging properties. Insome examples, the amplitude limiters 302 a and 302 b, and the filters304 a and 304 b are matched.

In some examples, when the error signal is minimized the signalgenerator output is effectively matched to the center frequency of theinput signal. For example, the error signal can be considered asminimized when its magnitude is zero or substantially close to zero(e.g., when the control signal has a magnitude that is negligible inrelation signal magnitudes measureable or usable by components of theCAREX 206). In some examples, the error signal is considered to beminimized when its magnitude is below a threshold value (e.g., an errortolerance threshold).

In some implementations, the CAREX 206 is adapted to extract carrierfrequencies from single sideband signals. In some examples, the CAREX206 includes a controller that offsets the output signal of the signalgenerator 308 by an appropriate offset frequency. For example, theoutput of the frequency generator 308 can be offset after it is fed backto the second frequency detector 306 b, so as to not adversely affectthe control signal. In some examples, the first frequency detector 306 acan be configured to determine a frequency offset based on the bandwidthof the input signal. In such examples, the first frequency detector 306a can adjust the detected frequency by the frequency offset.

FIG. 3B is a block diagram of an example frequency detector 306 inaccordance with implementations of the present disclosure. The frequencydetector 306 illustrated in FIG. 3B is an example quadrature-baseddetector circuit. The frequency detector 306 includes a phase shiftnetwork 350, a signal mixer 352, and a filter 354. The phase shiftnetwork 350 is a frequency sensitive circuit, such as an all passfilter, for example, that causes a phase shift in an input signal thatcorresponds with the frequency of the input signal. In other words, thephase shift network 350 causes a change in the phase angle of the inputsignal relative to the frequency of the input signal. In some examples,the phase shift network 350 is tuned to produce a nominal phase shift of90 degrees (e.g., quadrature to the input signal) for a nominal designfrequency (e.g., a 70 MHz IF for a communication system).

The signal mixer 352 can be, for example, a signal multiplier. Thesignal mixer 352 receives the input signal and an output signal from thephase shift network 350 as inputs. The filter 354 is a low pass filter.

Plot 360 shows example signals at various points in the frequencydetector 306. The input signal (Signal A) is passed to the phase shiftnetwork 350 and the signal mixer 352. Signal A is shown as a sinusoidfor simplicity, however, Signal A can be a modulated signal. Signal B isthe output of the phase shift network 350 and is phase shifted relativeto the input signal (Signal A). The value of the phase shift correspondsto the frequency of Signal A, and is nominally 90 degrees for a designfrequency. Deviations from the design frequency resulting in a phaseshift of Signal B that deviates from the nominal 90 degrees. The inputsignal (Signal A) is mixed with the output of the phase shift network350 (Signal B) to produce Signal C (e.g., Signal C=Signal A×Signal B).Signal C has a DC offset component corresponding to the phase differencebetween Signals A and B, and by extension, to the frequency of Signal A.The low pass filter 354 then removes the high frequency components ofSignal C leaving only the DC component (Signal D). The deviation ofSignal B's phase shift from the a nominal 90 degrees is exaggerated inplot 360 in order to clearly show the resulting DC output signal (SignalD).

FIG. 4A depicts a plot 400 of an example control signal 402 generated inan example CAREX 206. The plotted control signal 402 is an example ofthe input signal to the signal generator 308 of FIG. 3A. The plottedcontrol signal 452 is broken into several regions (406-410). The regionsillustrate a variations 404 in the control signal 402 as the inputsignal to the CAREX 206 is switched between several different inputsignals, each modulated using a different type of modulation. The inputsignal in region 406 is a QPSK modulated signal. The input signal inregion 408 is a QAM modulated signal. The input signal in region 410 isan unmodulated carrier signal. Each of the input signals in regions406-410 is applied to a 70 MHz carrier. The plot 400 illustrates therobustness of the CAREX 206 and its adaptability to extracting carriersignals from various input signals without regard to the types ofmodulation applied to the carrier signal.

FIG. 4B depicts a plot 450 of another example control signal 452generated in an example CAREX 206. The plotted control signal 452 is anexample of the input signal to the signal generator 308 of FIG. 3A. Theplotted control signal 452 is broken into several regions (456-460). Theregions illustrate transitions 454 of the control signal 452 as theinput signal to the CAREX 206 is switched between several differentinput signals, each having a different carrier frequency. The inputsignal in region 456 is a 67 MHz carrier signal. The input signal inregion 458 is a 73 MHz carrier signal. The input signal in region 460 isa 70 MHz carrier signal. The plot 450 illustrates the robustness of theCAREX 206 and its adaptability to extracting different frequency carriersignals. In some implementations, as shown, the CAREX 206 loop can bedesigned for a specific center frequency (e.g., 70 MHz as shown). Forexample, the design center frequency can be a specific carrier frequencyor IF of a communication system such as a satellite or radio frequency(RF) communication system, for example.

FIG. 5 depicts a block diagram of an example TM signal receiver 106 inaccordance with implementations of the present disclosure. The TMreceiver 106 includes a carrier extraction portion (e.g., CAREX 506), aharmonic generation portion 504, a signal separation and extractionportion (SEPEX) device 512, and a TM demodulator 514. As in the TMtransmitter 104, the harmonic generation portion includes a secondharmonic generator 508 and a third harmonic generator 510. In addition,the TM receiver 106 can include a signal splitter 502 to split acombined input signal (e.g. combined signal 112 of FIG. 1) between theTM receiver 106 and a signal receiver for traditional modulated signals.

In operation, the TM receiver 106 receives a combined input signal andprovides the combined signal to both the CAREX 506 and SEPEX device 512.As described above in reference to the TM receiver 106, the CAREX 506extracts a carrier signal (f_(c)) from the combined signal, and thesecond harmonic generator 508 and third harmonic generator 510,respectively, generate second and third harmonics (2 f _(c) and 3 f_(c)) of the extracted fundamental carrier frequency (f_(c)). Both thecarrier signal (f_(c)) and second harmonic signal (2 f _(c)) areprovided to the SEPEX device 512. The third harmonic signal (3 f _(c))is provided to the TM demodulator 514.

The TM demodulation portion 504 separates and extracts the traditionallymodulated signal from the combined signal to obtain the TM modulatedsignal. The SEPEX device 512 provides the TM modulated signal to the TMdemodulator 514, which, demodulates the TM modulated signal to obtain abaseband data signal. The SEPEX device 512 separates and extracts the TMmodulated signal from the combined signal. In some implementations,before outputting the TM modulated signal, the SEPEX device 512heterodynes (e.g., up-shifts) the TM modulated signal to the thirdharmonic frequency (3 f _(c)) for demodulation. The SEPEX device 512 isdescribed in more detail below in reference to FIG. 6A.

The TM demodulator 514 uses the third harmonic signal (3 f _(c))provided by the third harmonic generator 210 as a reference signal forTM demodulation. The TM demodulator 514 demodulates the TM signal bysensing the time shifts between TM modulated carrier signal from theSEPEX device 512 and the third harmonic signal (3 f _(c)). In someexamples, the TM demodulator 514 can be a phase detection circuit. Insome implementations, the TM demodulator 514 detects the time shifts bydetermining a correlation between the TM modulated carrier signal andthe third harmonic signal (3 f _(c)) based on, for example, a product ofthe two signals.

FIG. 6A depicts a block diagram of an example TM signal SEPEX device 512in accordance with implementations of the present disclosure. The SEPEXdevice 512 can be implemented as a circuit in a device such as a TMreceiver, for example. In some implementations, the SEPEX device 512 canbe implemented as a standalone device for installation into in a largersystem (e.g., an application specific integrated circuit (ASIC) or fieldprogrammable logic array (FPGA)). In some implementations, the SEPEXdevice 512 can be implemented in software, for example, as a set ofinstructions in a computing device or a digital signal processor (DSP).

In operation, the SEPEX device 512 demodulates the traditionallymodulated signal from the combined signal. Because the TM modulation isnot detected by traditional signal demodulation, the resulting signaldoes not include the TM signal, but only the demodulated data signalfrom the traditional modulation signal. A “clean” (e.g., un-modulated)carrier is then re-modulated with the previously demodulated data signalfrom the traditional modulation signal. The SEPEX 512 computes thedifference between the combined signal and the re-modulated signal toobtain a TM modulated carrier signal. In other words, the SEPEX device512 removes a traditionally modulated signal from the combined signal bydemodulating the traditionally modulated signal, re-modulating a “clean”(e.g., un-modulated) carrier, and subtracting the re-modulated signalfrom the combined signal, thereby, leaving only the TM modulatedcarrier.

The SEPEX device 512 includes a signal demodulator 602, a signalmodulator 604, low-pass filters 606 a, 606 b, a summing circuit 608, adifference circuit 610, a delay circuit 612, a mixer 614, a bandpassfilter 616, and an amplitude limiter 618. The demodulator 602 is anon-TM signal demodulator, and the modulator 604 is a non-TM signalmodulator. That is, the demodulator 602 and modulator 604 aretraditional modulation type (e.g., AM, FM, PM, QAM, APSK, etc.)demodulator and modulator. The demodulator 602 and modulator 604 aredepicted as a complex (e.g., quadrature and in-phase) demodulator andmodulator, however, in some examples the demodulator 602 and modulator604 can be a simple (e.g., single phase) demodulator and modulator.

The operation the SEPEX device 512 is described below in more detail andwith reference to FIGS. 6A and 6B. FIG. 6B depicts frequency domainrepresentations of signals (A-F) at various stages of the SEPEX device512. The demodulator 602 receives the combined signal (A) (e.g. combinedsignal 112 of FIG. 1) as one input, and the carrier signal (f_(c)) fromthe CAREX 506 as a second input. The combined signal includes both atraditionally modulated signal and a TM modulated signal. As shown bysignal (A) in FIG. 6B, the combined signal includes frequency contentfrom both the TM modulated signal and the traditionally modulated signalcentered about the carrier frequency (f_(c)). The demodulator 602demodulates the traditional modulated signal from the combined signalproducing a baseband data signal. As noted above, because the TMmodulation is not detected by traditional signal demodulation, theresulting baseband data signal does not include a TM signal.

In the case of complex modulation, the demodulator 602 demodulates boththe in-phase and quadrature phase of the combined signal producing anin-phase and a quadrature phase baseband data signal. The low-passfilters 606 a and 606 b remove any extraneous signals or noise from thebaseband data signals, for example, harmonics introduced by thedemodulation process. The resulting baseband data signal, shown bysignal (B), includes only the frequency content from the traditionallymodulated signal centered at zero frequency (baseband). Morespecifically, a TM modulated signal does not exist at baseband, andthus, the TM modulated signal is removed by converting the traditionallymodulated signal to baseband.

The modulator 604 receives the baseband data signals (e.g., in-phase andquadrature phase signals) as a first input, and the carrier signal(f_(c)) from the CAREX 506 as a second input. The modulator 604re-modulates the un-modulated carrier signal (f_(c)) from the CAREX 506with the baseband data signals resulting in re-modulated carriers(re-modulated in-phase and quadrature phase carriers) having only thetraditionally modulated signal. The in-phase and quadrature phasere-modulated carriers are combined by the summing circuit 608 (signal(C)). FIG. 6B signal (C) shows the re-modulated signal again centeredabout the carrier frequency (f_(c)). In some examples, the carriersignal (f_(c)) may be phase shifted or delayed to account for delaysintroduced into the baseband data signals during the demodulation andfiltering process. This is to ensure that the resulting re-modulatedsignal is in phase with the combined signal.

The re-modulated signal is subtracted from the combined signal by thedifference circuit 610 removing the traditionally modulated signal fromthe combined signal. The resulting signal, show by signal (D), includesonly the TM modulated carrier signal (f_(c)). The combined signal isdelayed by the delay circuit 612 to account for delays introduced intothe re-modulated signal by the demodulation and re-modulation process.

The TM modulated signal is heterodyned (e.g., up-shifted) to the thirdharmonic (3 fc) by the mixer 614. The mixer 614 multiplies the TMmodulated signal with the second harmonic (2 f _(c)) of the carrier fromthe second harmonic generator 508 producing signal (E). Heterodyning theTM modulated carrier signal (f_(c)) with the second harmonic (2 fc)shifts the TM modulated signal to both the third harmonic (3 fc) and thenegative carrier frequency (−fc) (e.g., a phase inverted version of theTM modulated signal at the carrier frequency). The bandpass filter 616removes the phase inverted TM signal at the carrier frequency leavingonly the TM modulated third harmonic (3 fc) (signal (F)), and theoptional amplitude limiter 618 removes any variations in the amplitudeof the TM modulated third harmonic signal.

In some examples, the SEPEX device 512 can include multiple differenttypes of demodulators 602 and modulators 604. For example, the SEPEXdevice 512 can include FM, PM, and QAM demodulators 602 and modulators604. In such examples, the SEPEX device 512 can also include a controldevice that detects the type of traditional modulation on input signal,and sends the input signal to the appropriate set of demodulator andmodulator.

Although the SEPEX device 512 is described in the context of separatingand extracting a TM modulated signal from a traditionally modulatedsignal, in some implementations, the SEPEX device 512 can be modified toseparate two traditionally modulated signals such as separatingnon-quadrature modulated signals (e.g., in-phase modulated signal) andquadrature modulated signals. For example, a non-quadrature modulatedsignal could be separated and extracted from a combined I/Q modulatedsignal by modifying the SEPEX device 512 shown in FIG. 6A such that onlythe quadrature modulated signal is demodulated and demodulated bydemodulator 602 and modulator 604.

FIG. 7 depicts an example process 700 for extracting a carrier frequencyfrom an input signal that can be executed in accordance withimplementations of the present disclosure. In some examples, the exampleprocess 700 can be provided as computer-executable instructions executedusing one or more processing devices (e.g., a digital signal processor)or computing devices. In some examples, the process 700 may be hardwiredelectrical circuitry, for example, as an ASIC or an FPGA device.

A center frequency of an input signal is detected (702). For example,the center frequency can be detected based on frequency side lobes ofthe input signal. In some examples, the input signal can include thecarrier signal modulated with the modulation signal. In some examples,the input signal is a carrier signal modulated with a traditionalmodulation signal and a TM modulation signal. A frequency of a secondsignal is detected (704). For example, the second signal may be theoutput of a single generator such as, for example, a VCO or a VCXO. Adifference signal (e.g., control signal) is determined based on thecenter frequency of the input signal and the frequency of the secondsignal (706). For example, the difference signal represents a differencein frequency between the center frequency of the input signal and thefrequency of the second signal. In some examples, difference signal is aDC voltage signal.

The frequency of the second signal is modified based on the differencesignal to provide the carrier signal of the input signal (708), and thesecond signal is outputted as the carrier signal from the deviceperforming the process 700 (710). For example, a difference signal canbe a control signal for the signal generator and can cause the singlegenerator to adjust the frequency of its output signal. The frequency ofthe second signal modified until it is matched to the center frequencyof the input signal. In some examples, the frequency of the secondsignal is matched to the center frequency of the input signal when thedifference signal reaches a minimum value. In some examples, the minimumvalue may be a threshold value indicating that the difference betweenthe frequency of the second signal in the center frequency of inputsignal is within an allowable tolerance. In some examples, the minimumvalue may be a magnitude of the different signal voltage that is belowthe threshold minimum voltage magnitude.

FIG. 8 depicts an example process 800 for separating TM signals frominput signals that can be executed in accordance with implementations ofthe present disclosure. In some examples, the example process 800 can beprovided as computer-executable instructions executed using one or moreprocessing devices (e.g., a digital signal processor) or computingdevices. In some examples, the process 800 may be hardwired electricalcircuitry, for example, as an ASIC or an FPGA device.

An input signal including a carrier signal modulated with a firstmodulation signal and a second modulation signal is received (802). Forexample, the first modulation signal may be a traditional type ofmodulation signal such as, for example, FM, AM, PM, QAM, APSK, etc. Thesecond modulation signal may be a TM modulation signal. The firstmodulation signal is demodulated from the input signal (804). Forexample, the first modulation signal can be demodulated usingtraditional the modulation techniques. Because traditional demodulationtechniques do not recognize TM modulation, the resulting demodulatedfirst modulation signal will not include the TM modulation signal.

The carrier signal is re-modulated using the demodulated firstmodulation signal to generate a third signal (806). For example, thethird signal includes an un-modulated carrier signal modulated with thefirst modulation signal. The un-modulated carrier signal has the samefrequency as the carrier of the input signal. The first modulationsignal is removed from the input signal by subtracting the third signalfrom the input signal (808) to extract the second modulation signal(e.g., the TM modulation signal) from the input signal. In someexamples, the input signal must be delayed an appropriate amount of timeto ensure that it is in phase with the third signal. That is, due to thedemodulation and re-modulation process the third signal may be out ofphase with the original input signal. Thus, before subtracting the thirdsignal from the input signal, the input signal can be delayed anappropriate amount of time. The extracted second modulation signal isprovided to a signal demodulator (810). For example, an extracted TMmodulated signal can be provided to a TM signal demodulator fordemodulation.

Implementations of the subject matter and the operations described inthis specification can be realized in digital electronic circuitry, orin computer software, firmware, or hardware, including the structuresdisclosed in this specification and their structural equivalents, or incombinations of one or more of them. Implementations of the subjectmatter described in this specification can be realized using one or morecomputer programs, i.e., one or more modules of computer programinstructions, encoded on computer storage medium for execution by, or tocontrol the operation of, data processing apparatus. Alternatively or inaddition, the program instructions can be encoded on an artificiallygenerated propagated signal, e.g., a machine-generated electrical,optical, or electromagnetic signal that is generated to encodeinformation for transmission to suitable receiver apparatus forexecution by a data processing apparatus. A computer storage medium canbe, or be included in, a computer-readable storage device, acomputer-readable storage substrate, a random or serial access memoryarray or device, or a combination of one or more of them. Moreover,while a computer storage medium is not a propagated signal; a computerstorage medium can be a source or destination of computer programinstructions encoded in an artificially generated propagated signal. Thecomputer storage medium can also be, or be included in, one or moreseparate physical components or media (e.g., multiple CDs, disks, orother storage devices).

The operations described in this specification can be implemented asoperations performed by a data processing apparatus on data stored onone or more computer-readable storage devices or received from othersources.

The term “data processing apparatus” encompasses all kinds of apparatus,devices, and machines for processing data, including by way of example aprogrammable processor, a computer, a system on a chip, or multipleones, or combinations, of the foregoing. The apparatus can includespecial purpose logic circuitry, e.g., an FPGA (field programmable gatearray) or an ASIC (application-specific integrated circuit). Theapparatus can also include, in addition to hardware, code that createsan execution environment for the computer program in question, e.g.,code that constitutes processor firmware, a protocol stack, a databasemanagement system, an operating system, a cross-platform runtimeenvironment, a virtual machine, or a combination of one or more of them.The apparatus and execution environment can realize various differentcomputing model infrastructures, such as web services, distributedcomputing and grid computing infrastructures.

A computer program (also known as a program, software, softwareapplication, script, or code) can be written in any form of programminglanguage, including compiled or interpreted languages, declarative orprocedural languages, and it can be deployed in any form, including as astand-alone program or as a module, component, subroutine, object, orother unit suitable for use in a computing environment. A computerprogram can, but need not, correspond to a file in a file system. Aprogram can be stored in a portion of a file that holds other programsor data (e.g., one or more scripts stored in a markup languagedocument), in a single file dedicated to the program in question, or inmultiple coordinated files (e.g., files that store one or more modules,sub-programs, or portions of code). A computer program can be deployedto be executed on one computer or on multiple computers that are locatedat one site or distributed across multiple sites and interconnected by acommunication network.

The processes and logic flows described in this specification can beperformed by one or more programmable processors executing one or morecomputer programs to perform actions by operating on input data andgenerating output. The processes and logic flows can also be performedby, and apparatus can also be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) or an ASIC(application-specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only memory ora random access memory or both. Elements of a computer can include aprocessor for performing actions in accordance with instructions and oneor more memory devices for storing instructions and data. Moreover, acomputer can be embedded in another device, e.g., a mobile telephone, apersonal digital assistant (PDA), a mobile audio or video player, a gameconsole, a Global Positioning System (GPS) receiver, or a portablestorage device (e.g., a universal serial bus (USB) flash drive), to namejust a few. Devices suitable for storing computer program instructionsand data include all forms of non-volatile memory, media and memorydevices, including by way of example semiconductor memory devices, e.g.,EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internalhard disks or removable disks; magneto-optical disks; and CD-ROM andDVD-ROM disks. The processor and the memory can be supplemented by, orincorporated in, special purpose logic circuitry.

While this specification contains many specific implementation details,these should not be construed as limitations on the scope of anyimplementation of the present disclosure or of what can be claimed, butrather as descriptions of features specific to example implementations.Certain features that are described in this specification in the contextof separate implementations can also be implemented in combination in asingle implementation. Conversely, various features that are describedin the context of a single implementation can also be implemented inmultiple implementations separately or in any suitable sub-combination.Moreover, although features can be described above as acting in certaincombinations and even initially claimed as such, one or more featuresfrom a claimed combination can in some cases be excised from thecombination, and the claimed combination can be directed to asub-combination or variation of a sub-combination.

Similarly, while operations are depicted in the drawings in a particularorder, this should not be understood as requiring that such operationsbe performed in the particular order shown or in sequential order, orthat all illustrated operations be performed, to achieve desirableresults. In certain circumstances, multitasking and parallel processingcan be advantageous. Moreover, the separation of various systemcomponents in the implementations described above should not beunderstood as requiring such separation in all implementations, and itshould be understood that the described program components and systemscan generally be integrated together in a single software product orpackaged into multiple software products.

Thus, particular implementations of the subject matter have beendescribed. Other implementations are within the scope of the followingclaims. In some cases, the actions recited in the claims can beperformed in a different order and still achieve desirable results. Inaddition, the processes depicted in the accompanying figures do notnecessarily require the particular order shown, or sequential order, toachieve desirable results. In certain implementations, multitasking andparallel processing can be advantageous.

What is claimed is:
 1. A system comprising: at least one processor; anda data store coupled to the at least one processor having instructionsstored thereon which, when executed by the at least one processor,causes the at least one processor to perform operations comprising:detecting a center frequency of an input signal, the input signalcomprising a carrier signal modulated with a modulation signal;detecting a frequency of a second signal; determining a differencesignal between the center frequency of the input signal and thefrequency of the second signal; modifying the frequency of the secondsignal based on the difference signal to provide the carrier signal; andoutputting the carrier signal.
 2. The system of claim 1, wherein the atleast one processor includes one of: a field programmable gate array(FPGA), an application specific integrated circuit (ASIC), or a digitalsignal processor (DSP).
 3. The system of claim 1, wherein the carriersignal is suppressed in the input signal.
 4. The system of claim 1,wherein the second signal is generated by a signal generator.
 5. Thesystem of claim 4, wherein the signal generator is a voltage controlledoscillator coupled to the at least one processor.
 6. The system of claim4, wherein modifying the frequency of the second signal comprises usingthe difference signal to control the signal generator.
 7. The system ofclaim 1, wherein the operations comprise: limiting an amplitude of theinput signal; and limiting an amplitude of the second signal.
 8. Thesystem of claim 1, wherein the method further comprises amplifying thedifference signal.
 9. The system of claim 1, wherein detecting thecenter frequency of the input signal comprises: multiplying the inputsignal with a third signal to create a fourth signal; and converting thefourth signal into a direct current (DC) voltage signal representing thecenter frequency of the modulated signal by integrating the fourthsignal.
 10. The system of claim 9, wherein the third signal is a phaseshifted version of the input signal.
 11. The system of claim 1, whereindetecting the frequency of the second signal comprises: multiplying thesecond signal with a third signal to create a fourth signal; andconverting the fourth signal into a direct current (DC) voltage signalrepresenting the frequency of the second signal by integrating thefourth signal.
 12. The system of claim 11, wherein the third signal is aphase shifted version of the second signal.
 13. A non-transient computerreadable storage medium storing instructions that, when executed by atleast one processor, cause the at least one processor to performoperations comprising: detecting a center frequency of an input signal,the input signal comprising a carrier signal modulated with a modulationsignal; detecting a frequency of a second signal; determining adifference signal between the center frequency of the input signal andthe frequency of the second signal; modifying the frequency of thesecond signal based on the difference signal to provide the carriersignal; and outputting the carrier signal.
 14. The medium of claim 13,wherein modifying the frequency of the second signal comprises using thedifference signal to control a signal generator that generates thesecond signal.
 15. The medium of claim 13, wherein the operationscomprise: limiting an amplitude of the input signal; and limiting anamplitude of the second signal.
 16. The medium of claim 13, wherein themethod further comprises amplifying the difference signal.
 17. Themedium of claim 13, wherein detecting the center frequency of the inputsignal comprises: multiplying the input signal with a third signal tocreate a fourth signal; and converting the fourth signal into a directcurrent (DC) voltage signal representing the center frequency of themodulated signal by integrating the fourth signal.
 18. The medium ofclaim 17, wherein the third signal is a phase shifted version of theinput signal.
 19. The medium of claim 13, wherein detecting thefrequency of the second signal comprises: multiplying the second signalwith a third signal to create a fourth signal; and converting the fourthsignal into a direct current (DC) voltage signal representing thefrequency of the second signal by integrating the fourth signal.
 20. Themedium of claim 19, wherein the third signal is a phase shifted versionof the second signal.